Prosthetic implants, which can be made from natural or synthetic materials, include, for example, heart valves, vascular grafts, urinary bladder prostheses, and tendon prostheses. Bioprostheses (i.e., prostheses derived from natural tissue) are typically preferred over synthetic or mechanical prostheses. For example, natural tissue valves are preferred over mechanical valves because tissue valves stimulate the natural flow of the blood better than mechanical valves. Also, no blood anticoagulants are needed when natural tissue valves are used.
Tissue heart valve prostheses are typically made from either porcine aortic valves or bovine pericardium; ostrich or kangaroo are successful alternatives. Such valves are typically made by pretreating the tissue with glutaraldehyde or other crosslinking agents, as discussed below, and sewing the tissue into a flexible metallic alloy or polymeric stent. Such animal tissues mainly consist of collagen and elastin. These components provide the tissues, particularly heart valves, with their needed mechanical strength and flexibility.
Collagen-based materials, including whole tissue, are finding increased use in the manufacture of biomedical devices, such as prosthetic implants. This is particularly true for heart valves. Collagen is the major structural protein in higher vertebrate organisms forming extracellular fibers or networks in practically every tissue of the body, including skin, bone, cartilage, and blood vessels. Its molecular structure is highly conserved across many species lines, which makes it an ideal implant material with good biocompatibility. In medical devices, collagen provides a more physiological, isotropic environment that has been shown to promote the growth and function of different cell types, facilitating the rapid overgrowth of host tissue after implantation.
Basically, three types of collagen-based materials can be identified, based on the differences in the purity and integrity of the collagen fiber bundle network initially present in the material. The first type includes whole tissue including non-collagenous substances or cells. As a result of using whole tissue, the naturally occurring composition and the native strength and structure of the collagen fiber bundle network are preserved. Whole tissue xenografts have been used in construction of heart valve prostheses, and also in vascular prostheses. However, the presence of soluble proteins, glycoproteins, glycosaminoglycans, and cellular components in such whole tissue xenografts may induce an immunological response of the host organism to the implant.
The second type of collagen-based material includes only the collagen matrix without the non-collagenous substances. The naturally occurring structure of the collagen fiber bundle network is thus preserved, but the antigenicity of the material is reduced. The fibrous collagen materials obtained by removing the antigenic non-collagenous substances will generally have suitable mechanical properties.
The third type of collagen-based material is purified fibrous collagen. Purified collagen is obtained from whole tissue by first dispersing or solubilizing the whole tissue by either mechanical or enzymatic action. The collagen dispersion or solution is then reconstituted by either air drying, lyophilizing, or precipitating out the collagen. A variety of geometrical shapes like sheets, tubes, sponges or fibers can be obtained from the collagen in this way. The resulting materials, however, do not have the mechanical strength of the naturally occurring fibrous collagen structure.
A major problem in the use of collagen-based materials, and especially whole tissue xenografts in which the donor and recipient are phylogenetically distant, for implantation is that these materials are prone to hyperacute rejection. This is a rapid and violent rejection reaction that leads to the destruction of the xenograft. Hyperacute rejection appears to be triggered by components of natural immunity, most notably natural antibodies and complement.
In order to use collagen-based materials in manufacturing medical devices, particularly bioprosthetic implants, their durability and in vivo performance typically need to be improved. This can be done by crosslinking the material. Crosslinking of collagen-based materials is used to suppress the antigenicity of the material in order to prevent the hyperacute rejection reaction. In addition, crosslinking is used to improve mechanical properties and enhance resistance to both mechanical and proteolytic degradation.
Several chemical crosslinking methods for collagen-based materials are known. These methods typically involve the reaction of a bifunctional reagent (i.e., a spacer) with the amine groups of lysine or hydroxylysine residues on different polypeptide chains or the activation of carboxyl groups of glutamic and aspartic acid residues followed by the reaction with an amine group of another polypeptide chain to give an amide bond. For example, glutaraldehyde (GA), which is a bifunctional aldehyde, or diisocyanates bridge amine groups on two adjacent polypeptide chains to form crosslinks.
Another method of crosslinking involves the formation of an acyl azide. The acyl azide method involves the activation of carboxyl groups in the polypeptide chain. The activated groups form crosslinks by reaction with collagen amine groups of another chain.
Also, water-soluble carbodiimides can be used to activate the free carboxyl groups of glutamic and aspartic acid moieties in collagen. Activation of the carboxyl groups with carbodiimides, such as 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide.HCl (EDC), gives O-acylisourea groups. A condensation reaction by nucleophilic attack of a free amine group of a (hydroxy)lysine residue with urea as a leaving group results in formation of an amide crosslink. The O-acylisourea can also be hydrolyzed or rearranged to an N-acylurea, which is much more stable and will not react to form a crosslink. Addition of N-hydroxysuccinimide (NHS) prevents this rearrangement, however. In the presence of NHS, the O-acylisourea can be converted to an NHS activated carboxyl group, which also can react with a free amine group to form a crosslink.
U.S. Pat. No. 6,166,184 discloses a method for making a bioprosthetic device made of collagen-based material that includes: blocking at least a portion of the collagen amine groups with a blocking agent; activating at least a portion of the collagen carboxyl groups after blocking at least a portion of the collagen amine groups to form activated carboxyl groups; and contacting the activated collagen carboxyl groups with a polyfunctional spacer (preferably a bifunctional spacer) to crosslink the collagen-based material. Although the resultant material has a generally high degree of crosslinking and a generally high resistance towards enzymatic digestion, and crosslinked material maintains a relatively high degree of flexibility without substantial stiffening over time, there is still a need for methods of crosslinking collagen-based materials that are even more flexible.